1. Field of the Invention
The present invention relates to a process for preparing an all-solid battery such as a lithium ion secondary battery.
2. Description of the Related Art
Lithium ion secondary batteries are composed of a positive electrode, negative electrode, an electrolyte (solid electrolyte) and a separator etc. have a light weight, a large capacity and high charge-discharge performance, and are widely used in fields of mobile devices such as laptop computer and cellular phone and field of automobiles. Various studies have been done for establishing larger capacity and higher charge-discharge performance.
The rate determining factor to enlarge the capacity and to enhance the charge-discharge performance is a rate of reaction of an electrolyte with a positive electrode active material contained in the positive electrode and a negative electrode active material contained in the negative electrode. However, because a lithium ion conductivity of the electrolyte is low, it is important to make a distance between the positive electrode and the negative electrode shorter, and to increase areas of the positive and negative electrodes as large as possible, especially to enlarge contact areas of the electrolyte with the positive and negative electrode active materials.
Considering this viewpoint, for example, JP2011-70788 A proposes, in order to present an all-solid battery to achieve low cost, high safety, high energy density-high performance, a process for preparing an all-solid battery which includes three-dimensional electrodes containing active materials of concavo-convex structure.
Namely, in JP2011-70788 A (claim 1, etc.), there is proposed a process for preparing an all-solid battery which includes a first active material layer forming step by forming the first active material layer having a given concavo-convex pattern by applying a coating liquid containing a first active material to a substrate, after the first active material layer forming step, a solid electrolyte layer forming step by applying a coating liquid containing a polymer electrolyte on the integrated substrate in which the first active material layer is integrated on the surface of the substrate to form a solid electrolyte layer having convexity and concavity almost along with the concavo-convex pattern on the integrated substrate, and, after the solid electrolyte layer forming step, a second active material layer forming step by applying a coating liquid containing a second active material to the surface of the solid electrolyte layer to form a second active material layer having an almost flat surface on the opposite side to the side which contacts with the solid electrolyte layer.
Generally when preparing a conventional lithium ion secondary battery by using a solid (polymer) electrolyte, at first, a negative electrode active material or a positive electrode active material is applied to a current collector, and then is sufficiently dried under vacuum at around 90□ for about 5 hours. After that, a precursor layer of an electrolyte containing a polymeric electrolyte monomer and a lithium salt is formed thereon, and then is dried and polymerized by heating at around 100□ to form a gel-like electrolyte film. Consequently, an active material which forms a counter electrode is applied and dried to form a current collector, and then a lithium ion secondary battery is fabricated.
According to the conventional fabrication process of lithium ion secondary batteries, since the polymerization of the polymeric electrolyte monomer is conducted on the almost flat surface of the active material layer, even if the monomer may be fluidized during the polymerization reaction with heating, an almost homogeneous solid electrolyte layer having a flat surface can be obtained after cooling, thus there is no particular problem.
According to the technique proposed in JP2011-70788 A, a solid electrolyte layer having a high aspect ratio can be prepared. However, since the solid electrolyte layer has to be formed on the surface of the first active material layer having the concavo-convex pattern, even if the precursor film is formed along with the concavo-convex pattern of the first active material layer, there is a case that a solid electrolyte layer having a low aspect ratio is prepared, because the electrolyte is fluidized at the polymerization with heating due to its thermoplastic property to flow into the concave portions, followed by solidified as it is to form a solid electrolyte layer. In such a case, since the contact area between the obtained solid electrolyte layer and the second active material layer is lowered, there is a problem that the battery performances such as large capacity and high charge-discharge ability are sacrificed.
The object of the present invention is to provide a process for preparing a solid electrolyte layer having a high aspect ratio through simple process steps, and to give an all-solid battery such as lithium ion secondary battery being excellent in large capacity and high charge-discharge performance.